D. Schouten wrote:
>
> I have implemented overvoltage protection with an output fuse followed by a
> parallel zener too. Works perfect! I have bypassed the pass transistor (not
> from Roman's circuit but from an IC solution) and applied 180VDC at once on
> the input. The zeners (two 10V/5W types in series) stay healty but the
> 200mAT fuse blows at once.

I'm curious, why fuse the output and not the input?
Also, they are two big and expensive Zeners! Couldn't
you use a smaller zener clamped with a larger capacitor?
It's not as though you need 10W of zeners to handle 4W
of surge for a few mS.

You can even reduce the Zener surge current even more by
adding a resistor between the cap and zener, like this,
(if your load can tolerate a slight overvoltage)

psu out ----*-------*--
            |       |
            |       R
            |       |
      big   C       |
      cap   |       |
            |       Zener
            |       |
            |       |
gnd --------*-------*--

So the zener is normally held a volt or so below
conduction, and on surge fault the cap has to charge an
entire volt before the zener dissipates any power,
even then the cap must charge another volt before the
zener dissipates maximum (4W).

On surge fault the fuse will blow well before the zener
dissipates maximum due to the large inrush currents
into the cap during charging those couple of volts. :o)

> My DC-DC circuit output's 15VDC but needs a minimum load current of 20mA to
> stay in continuous mode, or else the output voltage will rise almost
> linearly with Vin. So I thought, instead of using a 'dump' resistor load and
> trowing away efficiency, why not use the 20V zener clamp on the output to
> 'regulate' at 20V when there is no load applied to the converter? This
> worked ok, so now my overvoltage circuit solves the minimum load requirement
> too :-)

That's a smart way of doing it. :o) And like I said
above, as you have 5 volts or so between normal running
and the fault turning the zener on, i'm sure you can
reduce the zener size a LOT.
-Roman

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